FIG. 1a schematically illustrates an optical transmitter 2 connected to an optical receiver 4 via an all-optical path 6. A plurality of optical amplifiers 8 and dispersion compensator modules (DCMs) 10 can be installed along the path in order to increase signal reach. Without these DCMs, dispersion would rise as shown by the dashed line 12 in the accompanying dispersion profile. By inserting DCMs, a parabolic dispersion profile 14 can be achieved, which maximizes signal reach with this type of system.
However, in order to perform optical switching (or adding or dropping of wavelengths), the dispersion compensation of the optical path must follow a suitable compensation profile, which enables optical connection of multiple paths together and allowing wavelengths to be exchanged across the multiple paths. However, the adoption of such a dispersion profile reduces the overall available system margin, thus reducing the optical signal reach. For example, consider the simple schematic network shown in FIG. 1b having four nodes 20 and an optical switch (or cross-connect) 22. While dispersion can be compensated for paths A-B and C-D in an ideal method, if an optical signal on path A-B is to be switched onto path C-D, the dispersion profiles of paths A-B and C-D must enable this optical interconnection at the switch 22. This has for effect of reducing the signal reach along paths A-B and C-D
One prior-art solution to this problem is simply to shorten each link and regenerate the optical signal at the cross-connect 22 by performing optical-electrical-optical (OEO) regeneration, which diminishes overall system performance.
Another prior-art solution is to use dispersion compensation modules to reduce the dispersion to zero at the end of every span (as shown by dashed line 16 in FIG. 1a), thus increasing the number of locations where optical switching can be performed. However, this arrangement severely compromises signal reach.
Although some degree of control over the dispersion profile of an optical link can be achieved using adaptive dispersion compensation modules, these adaptive DCMs only control dispersion over a limited range.
Digital electrical-domain pre-compensation of optical signals is described in Applicant's co-pending U.S. patent applications Ser. Nos. 10/262,944, filed Oct. 3, 2002; 10/307,466 filed Dec. 2, 2002; and 10/405,236 filed Apr. 3, 2003; and International Patent Application No. PCT/CA03/01044 filed Jul. 11, 2003. These co-owned patent applications, the contents of which are hereby incorporated by reference, describe techniques for compensating both linear and nonlinear impairments in an optical path by pre-distorting an input signal x(t), in the electrical domain, and then using the thus predistorted signal to drive an optical modulator. Because compensation is implemented in the electrical domain, virtually any arbitrary compensation function can be implemented. This enables dispersion (and other optical impairments) to be compensated, without requiring optical dispersion compensation modules (DCMs) within the path. Elimination of DCMs has the additional advantage that it reduces the system gain required to obtain a desired signal reach, thereby enabling fewer (or lower performance) amplifiers. Furthermore, electrical-domain compensation facilitates system evolution, because changes in path equipment and/or performance parameters can readily be accommodated through suitable adjustment of the compensation function.
As illustrated schematically in FIG. 2, an optical transmitter 100 is adapted to perform electrical-domain pre-compensation of optical signals by pre-distorting an electrical signal which, when converted to a corresponding optical signal and transmitted through an optical path, substantially mitigates optical impairments incurred over the path. Within the optical transmitter 100 is a complex driver 102 which includes a signal processor (not shown) which receives the input data signal x(t), e.g. a digital data stream, and uses a compensation function C[ ] to compute successive multi-bit in-phase and quadrature values representing successive loci of the end-point of a desired or target optical E-field vector. The complex driver 102 also includes a linearizer (not shown) which uses the multi-bit loci to synthesize a pair of multi-bit digital drive signals which are then converted into analog (RF) signals by respective high-speed multi-bit Digital-to-Analog  Converters (DACs), such as the DACs described in U.S. Pat. No. 6,781,537 entitled “High Speed Digital To Analog Converter”, the contents of which are hereby incorporated by reference. The analog signals are then amplified and optionally band-pass filtered to generate the drive signals supplied to an E/O converter 104 (e.g. a Mach-Zehnder interferometer). The digital drive signals are computed such that the drive signals supplied to the E/O converter 104 generates an optical E-field E0(t) that is a high-fidelity reproduction of the target E-field computed by the signal processor of the complex driver 102.
In general, the complex driver 102 is capable of implementing any desired mathematical function, which means that the compensation function C[ ] can be selected to compensate any desired signal impairments, including, but not limited to, dispersion, Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM), four-wave mixing and polarization dependent effects (PDEs) such as polarization dependent loss. In addition, the compensation function C[ ] can be dynamically adjusted for changes in the optical properties of the path, and for component drift due to aging.
To establish a pre-compensated connection over a new path, however, requires that the optical parameters or characteristics for the new path be known in advance (i.e. measured and/or computed before the new connection is established). This is problematic when attempting to swiftly establish a new optical connection (“at-speed”), i.e. in an interval of tens or hundreds of milliseconds.
Thus, it remains highly desirable to be able to quickly establish a new connection over a pre-compensated optical path.